1. Technical Field
The present invention relates to a scanning electron microscope device which scans a specimen with a focused electron beam, and detects reflection electrons and secondary electrons generated from the specimen. Then based on detection signals thereof, the device displays a two-dimensional scanning image of the specimen on an image display device such as a cathode ray tube (CRT display device), and measures the surface shape and the like of the specimen at a high definition and high resolution.
2. Description of the Related Art
Conventionally, a scanning electron microscope is used for observing the shape of circuit patterns or contact holes of submicron order, on a specimen such as a semiconductor device of an LSI or the like.
Recently, the integration of semiconductor devices has increased, and the size of circuit patterns and contact holes formed on the specimen has become microscopic, bringing about the requirement for a high resolution scanning electron microscope device for observing these shapes.
As a method of improving the resolution of such a scanning electron microscope device, a lens having a retarding field such as a magnetic-electrostatic compound objective lens comprising an electrostatic lens and a magnetic lens, is used as the objective lens, and aberrations of the objective lens are kept to a minimum by shortening the distance (operating distance) between the tip of the objective lens and the specimen.
With such a device, normally secondary electrons generated from the specimen as a result of a primary electron beam irradiated onto the specimen are detected with a detector to thereby obtain a scanning image. However arranging such a magnetic-electrostatic compound objective lens close to the specimen can cause discharge or the like due to the influence from the retarding field thereof formed close to the specimen. Moreover, there is the possibility of a loss in optical performance due to disturbances occurring in the retarding field caused by the arrangement of the detector.
For this reason, with conventional electron microscope devices, it is necessary to arrange the detector at a location away from the magnetic-electrostatic compound objective lens.
Moreover, in the case of observing in a specimen, the shape of the bottom surface of a contact hole or the like having a large aspect ratio (the size of the depth compared to the width), since the secondary electrons generated from the bottom surface of the contact hole have a low energy, they collide with the inner wall of the contact hole and do not come out from the specimen surface. For this reason observation of the contact hole using secondary electrons is difficult.
Therefore, in observation of such locations, observation is performed using only reflection electrons.
Moreover, with an objective lens having a retarding field such as the abovementioned magnetic-electrostatic compound objective lens, the retarding field formed by the magnetic-electrostatic compound objective lens, so that the energy thereof is increased accelerates the secondary electrons generated from the specimen.
Therefore, with the conventional detection method, the detection becomes even more difficult.
On the other hand, a method is considered such as where as shown in FIG. 6, a Wine filter 6 in which an electric field and magnetic field are combined, is arranged on the optical axis for detecting the reflection electrons and secondary electrons 7 generated from the specimen 4, and only electrons of a specific energy are guided away from the optical axis, and then subjected to an electric potential close to the detector so that the reflection electrons and secondary electrons are attracted to the detector side.
Here numeral 1 denotes a secondary electron detector, 2 denotes an electromagnetic lens, 3 denotes an electrostatic lens, 4 denotes a specimen, and 5 denotes a deflecting coil.
Furthermore, as another conventional method, as shown in FIG. 7, a method is carried out for impinging the secondary electrons 7 accelerated by the abovementioned retarding field onto another target 8, and attracting the secondary electrons generated from this target 8 to a detector 1 side arranged away from the optical axis. In FIG. 7, parts the same as those in FIG. 6 are denoted with the same symbol.
Of the above described methods, with the method which uses the Wien filter 6 (FIG. 6), there is the problem that in consideration of the influence exerted by the Wien filter 6 on the primary electron beam, is also necessary to superimpose an auxiliary electromagnetic field, causing a worsening of aberration.
Moreover, with the latter shown method (FIG. 7) of impinging on a target, there is the problem that the strength of the signals which can be detected is very much smaller compared to the strength of the signals for the case where the secondary electrons generated from the specimen are detected by a direct detector.
Furthermore, in observing the bottom face of the contact hole having a high aspect ratio as mentioned above, a scanning image from reflection electrons is used. However a part of these reflection electrons impinge on the inner wall of the contact hole so that secondary electrons are generated from the inner wall.
Accordingly, with the conventional method, it is difficult to separate the reflection electrons from the secondary electrons. Therefore there is the problem that when observing the bottom face image of the contact hole, the reflection electron image and the secondary electron image are mixed so that the resultant image quality is impaired.
Therefore, it is an object of the present invention to provide a scanning electron microscope device which can separate and independently detect on an electron beam axis, reflection electrons and secondary electrons from a specimen, and which can obtain a good image, with a device of a simple construction.
As a means of solving these problems, a scanning electron microscope comprises: an electron beam source, an electron beam acceleration device for accelerating primary electrons generated by the electron beam source, a deflector for scanning and deflecting the accelerated primary electrons, a magnetic-electrostatic compound objective lens for focusing the scanned and deflected primary electrons onto a specimen mounted on a specimen support, a reflection electron detector for detecting reflection electrons generated from the specimen due to focusing and irradiating the primary electrons onto the specimen, a secondary electron detector for detecting secondary electrons generated from the specimen due to focusing and irradiating the primary electrons onto the specimen, and an image display device for displaying a specimen image from detection signals from each detector, wherein there is provided an aperture around an axis for passing an electron beam and secondary electrons around the axis through the reflection electron detector.
Moreover, the secondary electron detector is arranged around the axis, and between the reflection electron detector and the electron beam source.
Furthermore, the image display device performs arithmetic processing based on respective signals detected by the secondary electron detector and the reflection electron detector, to thereby form the specimen image.